synchronous Rectifier for hard switched full bridge smps

[phoenix87]

Hi,
I am planning on making a full bridge smps with a pfc in front.
As the output current is in excess of 45A , normal schottky based rectifiers are out of question as losses will be quite high. So i thought of adopting synchronous rectification . I am new to synchronous rectification , so i started looking forapp notes to study it a bit more.But all the appication notes that i seem to find are for resonant converter . I am not opting for resonant converter at this point of time ,as my understanding of them is not that clear, so though of going
with good old hard switched full bridge smps driven by the TL494. what seems to confuse me is there is any special requirement to drive the SR in the secondary side.
Find attached schematic ......
Is my idea correct for driving the SR directly from the the control chip ?
Kindly have look and tell me if i am doing it wrong or any other modification is required.


Regards

 

[mtwieg]

Keep in mind that the phase shifted full bridge topology is often referred to as ZVS, but it is not truly a resonant converter like the LLC topology and others. ZVS just is just obtained by putting a little bit of dead time in the full bridge. It's a good way to improve efficiency but it's not nearly as complicated as fully resonant converters.

In any case, you cannot just use a TL494 to control this type of synchronous rectifier. Consider that the SR fets in your circuit must carry the freewheeling current of the output choke, meaning that one of them must always be on (when operating in CCM) in order to avoid diode conduction. In fact it is good to overlap the conduction of both SR FETs during the freewheeling period to reduce losses. This means that the PWM for the SR FETs must be very different from the primary FETs. The TL494 doen't generate these distinct waveforms, but maybe it could with a few external logic gates....

Reading this datasheet should be helpful.

 

[phoenix87]

Is it possible to use SR with hard switched converters .....

 

[mtwieg]

Sure, but there's no good reason to do so. The problems I described apply regardless of whether the circuit is hard switched or not.

 

[Orson Cart]

Ideally use an HF CT on the sec to detect current and turn the sec mosfets on, and then off when the current gets down to 1A say, there is a good IC from IR that does this without a CT, up to 200V mav on sec windings